Continuous stream ink jet print head droplet generator having backing member bridging divided vibrator

ABSTRACT

A continuous stream ink jet print head has a droplet generator with an elongate cavity for containing the ink, and nozzle orifices in a wall of the cavity for passing ink from the cavity to form jets. The nozzle orifices are disposed at spaced positions along the length of the cavity. An actuator for vibrating the ink in the cavity is provided such that each jet breaks up into ink droplets at the same predetermined distance from the wall of the cavity. The actuator is disposed on the opposite side of the cavity to the wall, includes a vibrator which is divided up along the length of the cavity at least partially into at least two parts, and includes a backing member disposed further from the cavity than the vibrator and secured to and bridging the parts into which the vibrator is at least partially divided. The vibrator is interposed in line between the backing member and the cavity.

BACKGROUND

This invention relates to a droplet generator for a continuous streamink jet print head.

More particularly the invention relates to such a generator comprising:an elongate cavity for containing the ink; nozzle orifices in a wall ofsaid cavity for passing ink from the cavity to form-jets, said nozzleorifices extending along the length of said cavity; and actuator meansfor vibrating the ink in said cavity such that each said jet breaks upinto ink droplets at the same predetermined distance from said wall ofthe cavity, said actuator means being disposed on the opposite side ofsaid cavity to said wall and comprising vibration means which is dividedup along the length of said cavity at least partially into at least twoparts. An example of such a generator is disclosed in U.S. Pat. No.4,587,528. In order that each jet breaks up at the same distance fromthe generator, it is necessary that the vibration of the actuator meanshas no component along the length of the ink cavity. The at leastpartial division of the vibration means of the actuator means inhibitssuch components.

SUMMARY OF THE INVENTION

According to the present invention there is provided a droplet generatorfor a continuous stream ink jet print head comprising: an elongatecavity for containing the ink; nozzle orifices in a wall of said cavityfor passing ink from the cavity to form jets, said nozzle orificesextending along the length of said cavity; and actuator means forvibrating the ink in said cavity such that each said jet breaks up intoink droplets at the same predetermined distance from said wall of thecavity, said actuator means being disposed on the opposite side of saidcavity to said wall and comprising vibration means which is divided upalong the length of said cavity at least partially into at least twoparts, characterised in that said actuator means further comprises abacking member disposed further from said cavity than said vibrationmeans and secured to and bridging said parts into which said vibrationmeans is at least partially divided, said vibration means beinginterposed in line between said backing member and said cavity.

Preferably, said actuator means further comprises a head on the oppositeside of said vibration means to said backing member and secured to andbridging said parts into which said vibration means is at leastpartially divided.

Preferably, said vibration means is divided up along the length of saidcavity into at least two spaced parts. Suitably, the number of spacedparts is three.

Preferably, said vibration means is made of piezoelectric material.

Preferably, the backing member and, when provided, the head are made ofan electrically conductive material. Suitably, the electricallyconductive material is brass for the backing member, and steel for thehead.

BRIEF DESCRIPTION OF THE DRAWINGS

A droplet generator in accordance with the present invention will now bedescribed, by way of example, with reference to the accompanyingschematic drawings, in which:

FIG. 1 is a front view of the generator;

FIG. 2 is a side view of the generator of FIG. 1;

FIGS. 3A, 3B and 3C illustrate respectively front, side and plan viewsof an actuator of the generator of FIG. 1;

FIG. 4 is a graph of the frequency response of the actuator of FIGS. 3A,3B and 3C with a backing member thereof removed;

FIG. 5 is a graph of the frequency response of the actuator of FIGS. 3A,3B and 3C with the backing member in place; and

FIGS. 6 and 7 illustrate respectively alternative actuators.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, the generator comprises an actuator 1 heldwithin a manifold 3 by means of a compliant element 5 and an O-ring 7. Aspacer 9 and a nozzle plate 11 define, and form walls for, an elongatecavity 13 below actuator 1 to which ink is supplied under pressure.Nozzle plate 11 contains a line of nozzle orifices 15.

Referring also to FIGS. 3A, 3B and 3C, actuator 1 comprises a steel head17, three spaced apart elements 19 of piezoelectric material secured tohead 17, and a brass backing member 21 secured to and bridging elements19. Piezoelectric elements 19 are driven by means of a single electricalconnection 20 to brass backing member 21 and the earthing of steel head17.

Actuator 1 has a resonant frequency at which all points across bottomface 23 of actuator 1 vibrate vertically in phase and with the sameamplitude, i.e. at which bottom face 23 is driven in contact with theink in cavity 13 in piston-like manner. At this frequency actuator 1exhibits a vertical longitudinal mode of vibration, without the addedinterference of waves generated in other directions in actuator 1, whichwould give rise to complex 30 vibrations across bottom face 23. Let thisresonant frequency be termed the longitudinal resonant frequency. Letthe other resonant frequencies of actuator 1, which do include the addedinterference of waves generated in other directions, be termed mixedresonant frequencies. Since at the longitudinal resonant frequency allpoints across bottom face 23 vibrate vertically in phase and with thesame amplitude, the wave imparted to the ink in cavity 13 will causeeach ink jet emanating from a nozzle orifice 15 to break up intodroplets at the same distance from nozzle plate 11.

That actuator 1 has a longitudinal resonant frequency is attributable tothe following. The following also contribute to this resonant frequencybeing sufficiently far away from the mixed resonant frequencies, ie.contributes to the longitudinal resonant frequency having a sufficientlybroad bandwidth on each side of it between it and its adjacent mixedresonant frequencies.

(i) The divided-up nature of the piezoelectric part of actuator 1generally inhibits horizontal components of vibration.

(ii) The physical coupling together of piezoelectric elements 19 attheir top and bottom faces by means of rigid elements 21, 17respectively, alleviates the consequences of variation in precisephysical size and operational properties between piezoelectric elements19, by averaging out their individual responses. In this connection,referring to FIG. 4, at the frequency of each of peaks 25, 27, 29, thevibration of actuator 1 is close to being, but is not entirely,longitudinal. Each of peaks 25, 27, 29 corresponds to a respective oneof piezoelectric elements 19. Peaks 25, 27, 29 are spread due to theaforementioned variation in precise physical size and operationalproperties between elements 19. Thus, it will be seen that there is noclear longitudinal resonant frequency. Referring to FIG. 5, at thefrequency of peak 31 the vibration of actuator 1 is entirelylongitudinal. Thus, it will be seen that backing member 21 averages outthe individual responses of piezoelectric elements 19 to provide a clearlongitudinal resonant frequency.

(iii) The presence of backing member 21 increases the flexibility intuning of actuator 1.

Actuator 1 must be tuned such that the longitudinal resonant frequencyis at the desired frequency of operation of the droplet generator. Thisis done by making the height of actuator 1 equal to half of thewavelength which corresponds to the desired frequency of operation. Inthis connection, it should be noted that the half wavelength will be acomposite half wavelength, since three different materials (steel,piezoelectric and brass) are present, having three different speeds ofsound therein. Further, actuator 1 must be tuned such that there are nomixed resonant frequencies too near to the longitudinal resonantfrequency.

The height of steel head 17 is often dictated/fixed by factors relatedto the general design of the ink jet print head. Thus, if backing member21 were not present, to achieve longitudinal resonance at a particularfrequency, would fix, and leave no leeway with regard to, the height ofpiezoelectric elements 19. Therefore, if there was present anundesirably close mixed resonant frequency, since the respective heightsof head 17 and elements 19 are fixed, there is no room for adjustment ofthese heights to tune actuator 1 so as to push the undesirably closeresonant frequency further away.

The presence of backing member 21 provides the required room foradjustment. Although the total height of actuator 1, and the height ofhead 17, are fixed, the respective heights of elements 19 and backingmember 21 may be adjusted to push further away the aforementionedundesirable resonant frequency. Of course, the combined height ofelements 19 and member 21, must such that when it is taken together withthe fixed height of head 17, the total height of actuator 1 is equal tohalf the composite wavelength corresponding to the desired frequency ofoperation.

It is to be appreciated that since the undesirable resonant frequenciesare ones with a horizontal component, to push these resonant frequenciesfurther away from the longitudinal resonant frequency requires backingmember 21 to span the width of actuator 1.

Referring to FIG. 6, in this alternative actuator, there is no steelhead, and the piezoelectric part comprises a single piece 41 ofpiezoelectric material, with a slot 43 cut therein, which extends fromthe side remote from cavity 13 so as to partially divide piece 41 intotwo parts 45, 47. A backing member 49 is secured to and bridges parts45, 47. Since piezoelectric piece 41 contacts the ink in cavity 13, aprotective coating therefor is required.

Referring to FIG. 7, in this alternative actuator, again there is nosteel head, again the piezoelectric part comprises a single piece 51 ofpiezoelectric material, and again a slot 53 is cut in piece 51 so as topartially divide it into two parts 55, 57. However, in this actuator theslot extends from the side which contacts the ink in cavity 13. Again aprotective coating is provided for the piezoelectric part as it contactsthe ink. Again a backing member 59 is secured to and bridges parts 55,57.

What is claimed is:
 1. A continuous stream ink jet print headcomprising: a droplet generator including an elongate cavity forcontaining the ink; nozzle orifices in a wall of said cavity for passingink from the cavity to form jets, said nozzle orifices being disposed atspaced positions along the length of said cavity; actuator means forvibrating the ink in said cavity such that each said jet breaks up intoink droplets at the same predetermined distance from said wall of thecavity, said actuator means being disposed on the opposite side of saidcavity to said wall, vibrating relative to said wall, and comprisingvibration means which is divided up along the length of said cavity atleast partially into at least two parts, characterized in that saidactuator means further comprises a backing member disposed further fromsaid cavity than said vibration means and secured to and bridging saidparts into which said vibration means is at least partially divided,said vibration means being interposed in line between said backingmember and said cavity; and charge and deflection electrodes forselectively charging and deflecting the droplets generated by saiddroplet generator depending on which droplets are to be used to print.2. A print head according to claim 1 wherein said actuator means furthercomprises a head on the opposite side of said vibration means to saidbacking member and secured to and bridging said parts into which saidvibration means is at least partially divided.
 3. A print head accordingto claim 2 wherein said backing member and said head are made of anelectrically conductive material.
 4. A print head according to claim 3wherein said backing member is made of brass and said head is made ofsteel.
 5. A print head according to claim 1 wherein said vibration meansis divided up along the length of said cavity into at least two spacedparts.
 6. A print head according to claim 5 wherein the number of partsis three.
 7. A print head according to claim 1 wherein said vibrationmeans is made of piezoelectric material.
 8. A print head according toclaim 1, wherein said backing member is attached to all parts of saidvibration means for providing a longitudinal resonant frequency at allpositions along the length of the ink cavity so that said actuator meansvibrates in phase and with one amplitude.
 9. A print head according toclaim 1, wherein said actuator means has a face which engages the ink insaid cavity, and wherein said backing member averages out the individualresponses of said parts into which said vibration means is at leastpartially divided to provide a resonant frequency at which all points onsaid face vibrate in phase and with the same amplitude.
 10. A continuousstream ink jet print head, comprising: a droplet generator including anelongate cavity for containing the ink; nozzle orifices in a wall ofsaid cavity for passing ink from the cavity to form jets, said nozzleorifices being disposed at spaced positions along the length of saidcavity; and actuator means for vibrating the ink in said cavity suchthat each said jet breaks up into ink droplets at the same predetermineddistance from said wall of the cavity, said actuator means beingdisposed on the opposite side of said cavity to said wall, vibratingrelative to said wall, and comprising vibration means which is dividedup along the length of said cavity at least partially into at least twoparts, characterized in that said actuator means further comprises abacking member disposed further from said cavity than said vibrationmeans and secured to and bridging said parts into which said vibrationmeans is at least partially divided, said vibration means beinginterposed in line between said backing member and said cavity andwherein said backing member averages out the individual responses ofsaid parts into which said vibration means is at least partially dividedto provide a resonant frequency at which all points on said face vibratein phase and with the same amplitude.